The present disclosure generally relates to vehicle suspensions, and more particularly relates to an improved vehicle suspension having a kingpin axis that is substantially free of constraints imposed by the vehicle body.
Kingpin axis is a commonly used term used to define the axis about which a vehicle's tire is steered and is generally defined by one upper and one lower rotating component of a vehicle suspension. The kingpin angle is the angle of the kingpin axis relative to a vertical axis of the vehicle laterally disposed at a center of the wheel mounted on the knuckle. The kingpin offset is the distance between this center and the kingpin axis.
The kingpin offset can be thought of as the moment arm of a force acting about the kingpin axis. Thus, a smaller kingpin offset is advantageous as it reduces the disturbance caused by acceleration, commonly called torque steer. Furthermore, reducing the kingpin angle improves the geometrical efficiency of the steering system, which leads to reduced power demands from the steering system.
One of the most widely used vehicle suspension systems is the MacPherson strut suspension assembly. In this type of suspension assembly, a strut assembly, which includes a coil spring and a shock absorber, extends from the upper end of a knuckle to the vehicle body and a lower arm extends from a lower end of the knuckle to the vehicle body. A ball joint is used to rotatably connect the knuckle to the lower arm, while the strut assembly is rotatably connected to the body, typically by means of a bearing. The kingpin axis of the MacPherson strut assembly is defined by the lower ball joint connecting the knuckle to the lower arm and an upper mounting location at which the upper end of the strut assembly is connected to the vehicle body.
While widely employed, the MacPherson strut suspension assembly has certain limitations. For example, the kingpin angle is often limited to a relatively large angle (e.g., 10-15 degrees) and the kingpin offset is generally limited to a relatively large dimension (e.g., 60-80 mm). A further limitation of the MacPherson strut suspension assembly is that the long virtual arm of the strut assembly and the lower arm makes it difficult for the designer to tune the amount of camber change that occurs during suspension stroking.
Another widely used vehicle suspension system is the double wishbone suspension. In this type of suspension assembly, two wishbone-shaped arms locate the wheel. More specifically, a lower arm has an outer end connected to a lower end of a knuckle via a ball joint and has its inner wishbone-shaped arms connected to the vehicle body. An upper arm similarly has an outer end connected to an upper end of the knuckle via a ball joint and has its inner wishbone-shaped arms connected to the vehicle body at locations flanking an upper end portion of the spring-damper assembly. The spring-damper assembly has a lower end connected to the lower arm at a location offset from the lower ball joint and has its upper end mounted to the vehicle body. The kingpin axis of the double wishbone suspension assembly is defined by the lower ball joint connecting the lower arm to the knuckle and the upper ball joint connecting the upper arm to the knuckle.
While more expensive and complex, double wishbone suspension assemblies generally afford more freedom of design, which allows better control of suspension parameters such as camber angle, toe angle, etc. Also, the kingpin angle can be reduced (e.g., to 8-11 degrees) and the kingpin offset can be reduced (e.g., to 45-60 mm), particularly as compared to the MacPherson suspension assembly. However, in addition to cost and complexity, the double wishbone suspension assembly can sometimes require more packaging space on the vehicle.
In both the MacPherson strut type and the double wishbone type suspension assemblies, the kingpin axis is dependent on the particular location of one or more suspension components connected to the vehicle body. As such, a designer of these suspensions has limited flexibility to alter the kingpin axis, kingpin angle, or kingpin offset due to the constraints of the vehicle body. Furthermore, the vehicle body structures for each suspension type are usually unique and not interchangeable.